Answer will be 30 Lets Try to see how it came
First of all it is a metal sheet of square shape of length 100 cm now 4 squares of side x cm are removed from four corners so now length of remaining metal sheet will be 100-2x and when it is folded it will become cuboid in which length will be 100-2x and breadth 100-2x and height will be x now Volume of this cuboid is= L X B X H=(100-2x)(100-2x)x=(100-2x)^2x cm^3 now cost of 1 cm^3 is x^2 so for entire cost will be (100-2x)^2x^3 now in order to find maximum perform differentiation. UV rule of differentitaion ….. it will be -4x+3(100-2x)=0 so -4x+300-6x=0 so 10x=300 so x is 30.